The amount of power a central processing unit (CPU) uses is defined as the product of the voltage across the CPU and the current flowing through the CPU. The current flowing through the CPU is proportional to the speed of the CPU. As semiconductor technologies evolve, CPU devices are allowed to operate at much higher speeds. Such higher speeds cause extra power consumption. In order to reduce power consumption and heat dissipation while keeping the same CPU performance, the CPU voltage has to be reduced so that the total amount of power the CPU draws will be reduced accordingly.
At the early stage of computer development, a CPU operated from a 5 volt (V) power supply. As new CPU design and manufacturing technologies have been employed, new generation CPU devices are capable of steadily operating from a voltage as low as approximately 0.9V. Such a low supply voltage allows CPU devices to be fabricated in a 1.8V CMOS process. On the other hand, some peripheral devices such as input/output (I/O) devices still operate from a higher voltage supply (e.g., 3.3V). When a logic signal is forwarded from an I/O device to a CPU, the mismatch between the supply voltages of two devices may cause a reliability issue. More particularly, the logic signal having a high voltage (e.g., 3.3V) may exceed the maximum voltage (e.g., 1.8V) to which the CPU is specified.
Conventional voltage level shifting devices are employed to shift a voltage level up when a logic signal is forwarded from a CPU to an I/O device and shift a voltage level down when a logic signal is sent from an I/O device to a CPU. A variety of voltage level shifting devices have been adopted to convert an input voltage signal to an output voltage signal within a range suitable for a device operating from a different supply voltage. For example, a resistive divider and an operational amplifier may form a high-to-low voltage level shifter. First, the resistive divider scales the input high voltage signal down to a range suitable for the operation amplifier having a low supply voltage. The operation amplifier generates a signal having a low voltage suitable for subsequent circuits.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.